Mapping anomalous dispersion of air with ultrashort mid-infrared pulses

We present experimental studies of long-distance transmission of ultrashort mid-infrared laser pulses through atmospheric air, probing air dispersion in the 3.6–4.2-μm wavelength range. Atmospheric air is still highly transparent to electromagnetic radiation in this spectral region, making it interesting for long-distance signal transmission. However, unlike most of the high-transmission regions in gas media, the group-velocity dispersion, as we show in this work, is anomalous at these wavelengths due to the nearby asymmetric-stretch rovibrational band of atmospheric carbon dioxide. The spectrograms of ultrashort mid-infrared laser pulses transmitted over a distance of 60 m in our experiments provide a map of air dispersion in this wavelength range, revealing clear signatures of anomalous dispersion, with anomalous group delays as long as 1.8 ps detected across the bandwidth covered by 80-fs laser pulses.